1. Technical Field
This invention relates generally to semiconductor technology, and more particularly, to semiconductor devices which incorporate a silicidation process.
2. Background Art
A common trend in modern integrated circuit manufacture is to produce transistors having feature sizes as small as possible. To achieve a high density integrated circuit, features such as the conductors, source and drain junctions, and interconnections to the junctions must be made as smallest possible. As feature sizes decrease, the sizes of the resulting transistors as well as the interconnections between transistors also decrease. Having smaller transistors allows more transistors to be placed on a single monolithic substrate. Accordingly, relatively large circuits can be incorporated on a single and relatively small die area. Furthermore, smaller transistors typically have lower turn on threshold voltages and faster switching speeds and consume less power in their operation. These features, in combination, allow for higher speed integrated circuits.
As semiconductor transistors are scaled to reduce their dimensions, a number of problems have been presented. For example, use of a very thin gate dielectric causes high gate current leakage, which diminishes device performance. Furthermore, as a transistor is scaled, a higher doping level is needed in the channel to reduce short channel effect, in order to ensure that the transistor properly turns off. This very high concentration of dopant in the channel decreases current drive and can lead to undesirable drain-to-channel tunneling current.
Furthermore, use of polysilicon gate technology, as is typical, carries with it additional problems. For example, polysilicon gates tend to suffer from polysilicon depletion or boron penetration effects, causing degradation in performance.
Additionally, a polysilicon gate has a fixed work function defined by a certain high level of doping of a particular specie or kind. For example, in a typical process, in an N type transistor wherein the gate, source and drain are doped with arsenic to a chosen (high) concentration, the resulting work function would be approximately 4.1 eV, while in a P type transistor, wherein the gate, source and drain are doped with boron to a chosen (high) concentration, the resulting work function would be approximately 5.0 eV. While such values are acceptable for typical devices, as such devices are scaled as described above, increasing the work function value to an extent in an N type device, and decreasing the work function value to an extent in a P type device, would allow a reduced level of doping concentration in the channel for the same threshold voltage, overcoming the problems associated with a high dopant level in the channel described above.
The use of metal in place of polysilicon as the gate of a transistor provides many advantages. For example, a typical such metal has a higher conductivity than polysilicon. Furthermore, there is the opportunity to choose a metal so that its particular work function is suited to the device, allowing reduction in the level of doping concentration in the channel. Additionally, the problems of polysilicon depletion and boron penetration are avoided, allowing one to use a thicker gate oxide with a metal gate, substantially reducing gate current leakage. However, proposed processes typically involve deposition of metal instead of polysilicon, requiring complex process integration schemes.
Therefore, a process and a device which overcome the above problems are needed.
In the present method, a semiconductor structure is fabricated by providing a substrate, and providing a dielectric on the substrate. A polysilicon body is provided on the dielectric, and a metal layer is provided on the polysilicon body. A silicidation process is undertaken to silicidize substantially the entire polysilicon body to form a gate on the dielectric. In an alternative process, a cap layer is provided on the polysilicon body, which cap layer is removed prior to the silicidation process. The polysilicon body is doped with a chosen specie prior to the silicidation process, which dopant is driven during the silicidation process toward the dielectric to form a gate portion having a high concentration thereof adjacent the dielectric, the type and conception of this specie being instrumental in determining the work function of the formed gate.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there are shown and described embodiments of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.